Multiple stage polymerization processes are known in the art as is the use of metallocene catalyst systems. Multiple stage polymerization processes, such as two-stage polymerization processes, are generally used to prepare block copolymers which contain rubbery materials. Two-stage polymerization process products may include propylene block copolymers. In some instances, the propylene/ethylene copolymer portion of these block copolymers may be rubbery. In these instances, these products may be more suitable for molding applications rather than films. In other instances, two or more metallocenes may be used for the preparation of isotactic propylene polymers.
For some applications, a polymer such a polypropylene is desired having a very broad molecular weight range, while maintaining a high level of crystallinity or isotacticity. Furthermore, for many applications such as capacitor films where a low level of electrical conduction is desirable, it is ideal to have a low level of residual metals such as titanium, aluminum and ions such as chlorine, which are present in polypropylenes produced using traditional Ziegler-Natta catalysts. Low levels of these metals and ions are usually achieved in these polymers by washing or “deashing” the polymer alter production. However, even after this washing process, the level of conductible metals and ions still remains at an undesirable level. What is needed is a homopolymer such as polypropylene that is highly isotactic with a broad molecular weight distribution that also has a low metal residuals prior to washing. No polypropylene disclosed in the prior art to date serves these purposes.
Related patents and patent applications include: U.S. Pat. Nos. 6,127,042, 6,090,325, 6,094,337, 5,280,074, 5,322,902, 5,346,925, 5,350,817, 5,483,002, and 5,724,222; U.S. patent application Ser. No. 09/293,565, now abandoned (assigned to the assignee of the present invention), and Canadian Patent Application No. 2,133,181. What is needed is a film material including the desirable characteristics of metallocene-catalyzed polymers, and having the improved molecular weight distribution and low metal residues.